Toxoplasmosis of the central nervous system can have potentially devastating long term effects. It is caused by Toxoplasma gondii and it is the second leading cause of hospitalizations (8%) and deaths (24%) among food borne pathogens in the US. Unfortunately, current available drugs have significant toxicity and have no effect over the bradyzoite/latent form while the impending threat of emergence of resistance to these drugs makes the discovery of new therapeutic targets a priority. In this proposal, we show that auranofin has in vitro activity against T. gondii and induces accumulation of reactive oxygen species in infected host cells while in vivo, it prevents death in 100% of chicken embryos (acute toxoplasmosis model) and modulates the host immune response preventing an overwhelming inflammatory reaction. Auranofin's likely target is the thiol-dependent anti-oxidant system as demonstrated by our own observations on Entamoeba histolytica (thioredoxin reductase), and on other parasites such as Leishmania infantum (trypanothione reductase), and Schistosoma mansoni (glutathione-thioredoxin reductase). The long term goal of this proposal is to elucidate the roles of the thiol-dependent antioxidant system in T. gondii biology which are largely unknown. The central hypothesis is that T. gondii thiol-dependent antioxidant system is essential for T. gondii biology, and is a viable drug target. Specific Aim 1 will determine that T. gondii thiol-dependent anti-oxidant enzymes (one thioredoxin reductase and two thioredoxin-dependent peroxidases) are auranofin targets. T. gondii knockout strains of these enzymes will be generated to corroborate these observations. Recombinant enzymes will be expressed and their pharmacokinetics of interaction with auranofin in vitro will be evaluated. Specific Aim 2 wil evaluate auranofin's in vivo activity in standard mouse models of acute and chronic toxoplasmosis and will measure auranofin concentrations in CNS by mass spectrometry to assess blood brain barrier penetration. Specific Aim 3 will evaluate the effect of auranofin on the host immune response given that it has an impressive effect on parasite clearance and decreasing host tissue inflammation in vivo chicken embryo model of acute toxoplasmosis. Tracking autophagy will be possible using GFP-LC3 mice which have been instrumental to study this cellular process. In vitro and in vivo assays will be conducted to determine autophagy activation and progression in the presence of auranofin and during T. gondii infection. This project has the potential of identifying a new anti-parasitic drug target, the T. gondii thiol-dependent anti-oxidant system, that can be exploited in drug development for the treatment of toxoplasma infection. Re-purposing of a FDA-approved drug with a proven safety profile can dramatically accelerate the time to clinical trials.